As an ion exchange membrane to be used for an alkali chloride electrolysis method in which an alkali chloride aqueous solution such as salt water is electrolyzed to produce an alkali hydroxide and chlorine, a membrane comprising a fluorinated copolymer having carboxylic acid functional groups or sulfonic acid functional groups has been known. The fluorinated copolymer is obtainable, for example, by copolymerizing a fluorinated monomer having a carboxylic acid functional group or a sulfonic acid functional group, such as a perfluorovinyl ether and a fluorinated olefin such as tetrafluoroethylene (hereinafter referred to as TFE).
As a polymerization method, an emulsion polymerization method, a solution polymerization method, a suspension polymerization method or a bulk polymerization method may, for example, be mentioned. After the fluorinated copolymer is obtained by such a polymerization method, a step is required to separate and recover the polymerization medium or the unreacted monomer from the obtained fluorinated copolymer, and specifically, the following methods have been known.
(1) A method of pouring a slurry obtained by solution polymerization to a poor solvent such as methanol to agglomerate the fluorinated copolymer (for example, Patent Document 1).
(2) A method of extracting the unreacted monomer from an emulsion obtained by emulsion polymerization by solvent extraction, and adding an acid to agglomerate the fluorinated copolymer (for example, Patent Document 2).
(3) A method of purging the pressure of the reactor after polymerization, and evacuate the reactor of air by means of a cold trap with stirring to recover the polymerization medium and the unreacted monomer (Patent Document 3).
However, in the methods (1) and (2), in order that the fluorinated monomer having a carboxylic acid functional group or a sulfonic acid functional group which is expensive, is completely recovered, it is necessary to carry out agglomeration operation repeatedly several times and to distill off the solvent used for agglomeration to recover the fluorinated monomer, thus leading to a high cost.
In the method (3), as the recovery of the polymerization medium and the unreacted monomer proceeds, the fluorinated copolymer is agglomerated in the reactor, whereby heat transfer from the reactor to the agglomerated fluorinated copolymer is inhibited, and the polymerization medium and the unreacted monomer contained in the agglomerated fluorinated copolymer are hardly heated. Thus, the fluorinated monomer having a carboxylic acid functional group or a sulfonic acid functional group which is expensive cannot completely be recovered in a short time.
Accordingly, a method for efficiently recovering the polymerization medium and the fluorinated monomer having a carboxylic acid functional group or a sulfonic acid functional group has been desired.